Thermal oxidiser

ABSTRACT

Thermal oxidiser ( 1 ) for the oxidation of a feed liquid (W) comprising an elongated retort ( 3 ) provided with a combustion gas input lance ( 5 ), a feed liquid input lance ( 7 ), one or more flow straighteners ( 11 ) and an exhaust gas outlet ( 13 ) wherein said retort ( 3 ) comprises an elongated wall ( 15 ) with a first end plate ( 17 ) fastened to a first end ( 19 ) thereof and a second end plate ( 21 ) fastened to the second, opposite end ( 23 ) thereof, combustion gas input lance ( 5 ) projects substantially horizontally through first end plate ( 17 ) into said retort ( 3 ) and feed liquid input lance ( 7 ) projects downwards though the retort wall into said retort ( 3 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a filing under 35 U.S.C. §371 and claims priority to international patent application number PCT/GB2006/003090 filed Aug. 18, 2006, published on Feb. 22, 2007, as WO 2007/020454, which claims priority to British patent application number 0516879.4 filed Aug. 18, 2005.

FIELD OF THE INVENTION

The present invention relates to thermal oxidation devices and methods of the type mentioned in the preambles of the independent claims for the oxidation of material to its most oxidised state.

BACKGROUND OF THE INVENTION

In order to reduce environmental impact, waste materials from manufacturing processes usually have to be treated before being released to the environment. In some cases this treatment requires the destruction of the waste and this can be achieved by thermal treatments such as oxidation, pyrolysis, gasification or plasma processes. In all cases it is necessary to control the production of exhaust gases and residues in order to minimise their quantity and harmfulness. Residues may be in the form of ash or slag containing harmful substances such as organic compounds, dioxins, mercury, cadmium, alkali metals and oxides, acids, salts. Inefficient or incomplete thermal treatments may lead to exhaust gas containing undesirable substances such as particulate matter, hydrogen chloride, oxides of sulphur, nitrogen and carbon, and dioxins, in addition to the harmful substances found in ash or slag mentioned above.

SUMMARY OF THE INVENTION

According to the present invention, at least some of the problems with the prior art are solved by means of a device having the features present in claim 1 and a method having the features mentioned in claim 3.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic cross sectional view of an embodiment of a thermal oxidiser in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows schematically, not to scale, a thermal oxidiser 1 in accordance with an embodiment of the present invention which is intended to convert all species in the feed liquid (W) and feed gas (G) to their most oxidised state. Thermal oxidiser 1 comprises an elongated retort 3 provided with a combustion gas input lance 5, a feed liquid input lance 7, a salt tray 9, one or more flow straighteners 11 and an exhaust gas outlet 13. Retort 3 comprises an elongated cylindrical high temperature, corrosion-resistant wall 15 with a first end plate 17 sealably fastened to a first end 19 and a second end plate 21 sealably fastened to the second, opposite end 23 of the wall. During use retort 3 is arranged with its longitudinal axis substantially horizontal and the interior of retort 3 is nominally divided into six zones in the following order, starting from the first end 19: Feed Gas Zone (FG), Feed Liquid Zone (FL), Oxidation Zone (0), Residence Zone (R), Adjustment Zone (A) and Cooling Zone (C).

Each zone is electrically heated by electrical heaters (not shown) controlled by an automated control system (not shown), which keeps the interior of the retort at the required temperature profile and counteracts heat losses.

Combustion gas input lance 5 projects substantially horizontally through first end plate 17 into the Feed Gas Zone (FG). Nitrogen gas is fed to the combustion gas input lance 5 at a controlled rate, where it is mixed with oxygen and injected into the interior of the retort 3. Optionally volatile waste gas material may be added to the nitrogen/oxygen mixture. Preferably the combustion gas contains between 50% and 90% oxygen, the remainder of the gas being nitrogen and, optionally, waste gas material. More preferably the combustion gas contains between 60% and 80% oxygen, the remainder of the gas being nitrogen and, optionally, waste gas material. Most preferably the combustion gas contains between 65% and 75% oxygen, the remainder of the gas being nitrogen and, optionally, waste gas material. In the Feed Gas Zone the combustion gas is heated to the destruction temperature which is the temperature required to ensure that complete oxidation of the waste takes place.

Feed liquid input lance 7 projects downwards through the retort wall 15 into the Feed Liquid Zone (FL), preferably perpendicular to the longitudinal axis of the retort. Feed liquid W, for example radioactive waste liquid is supplied together with nitrogen to the feed liquid input lance 7. Feed liquid input lance 7 is preferably water cooled by a cooling jacket 25 provided with a supply of cooling water. The Feed Liquid Zone is the region around the water cooled feed liquid input lance, where the temperature is maintained to reduce deposition of inorganic salts up to a maximum temperature permissible for the feed liquid input lance 7. The opening 25 in the tip 27 of feed liquid input lance 7 in the retort 3 may be surrounded by a lip 29 which reduces the tendency of the liquid waste to collect around the tip 27 and thereby reduces any build-up of solid deposits around the tip.

Salt tray 9 is positioned below feed liquid input lance 7 and extends towards the second end of the retort. The feed liquid enters into the top of the Oxidation Zone where the liquid drops vertically on to the impingement salt tray. This allows for the volatiles to boil off and oxidise and allows for the inorganic salt content to be mostly collected as a slag 31. There is a progressive accumulation of slag 31 within the salt tray which may be removed at periodic intervals.

The Oxidation Zone (O) is the zone where the combustion gas and feed liquid meet and where the majority of the oxidation of the feed liquid occurs. There is a relatively large thermal demand to achieve the required Destruction Temperature for the following Residence Zone.

The Residence Zone (R) is an isothermal section where the process gases are kept at the destruction temperature for the required residence time. Ceramic flow straighteners positioned here provide good mixing and uniform residence time. There are three reasons for the use of the flow straighteners:

1) Improved Heat Transfer

The flow straighteners significantly increase the solid heat transfer area within the retort. The walls radiate heat towards the flows straighteners then as the gas passes through the channels, convective heat transfer from the flow straighteners ensure that the gas passing through is always close to the wall temperature.

2) Promoting Uniform Flow Profile

Unstraightened flow through the retort would normally be substantially laminar in the Residence Zone. The flow straighteners provide sufficient pressure drop and mixing to break up the parabolic laminar flow profile. A laminar flow profile would halve the residence time for the centreline as the centreline velocity is twice that of the average. Therefore, the effectiveness of the residence zone is improved by using flow straighteners.

3) Radiation Shields

The flow straighteners help to block radiation shine paths due to the ratio of the cross sectional area to the length of the channels. This is desirable as it prevents radiation heating of thermocouples (not shown) used to measure temperatures in the retort. It is for this reason that the flow straighteners are preferably positioned around the internal thermocouples (not shown), and may be used in the Residence Zone and the Adjustment Zone.

The Residence Zone is electrically heated to maintain the Destruction Temperature throughout this zone. After exiting from the Residence Zone, the process gas enters the Adjustment Zone.

The Adjustment Zone is intended to minimise radiation losses from the Residence Zone to the Cooling Zone. This zone has an electrical heater with a variable set point so that the heating duties of the Residence Zone and Cooling Zone can been varied independently. This helps to ensure that the exit of the Residence Zone is maintained at the Destruction Temperature without compromising the operation of the Cooling Zone.

In the Cooling Zone, the temperature is reduced to a level suitable for the flange of the exhaust gas outlet 13, by heat losses through the insulation to the outside environment. An electrical heater trims the temperature in this zone to ensure that the gas temperature does not fall below a minimum value. This minimum temperature is to allow process connection to a downstream Quench Column (not shown), while keeping above the point where unwanted organic by-products form.

After the Cooling Zone the process gas leaves the retort via the exhaust gas outlet and may be injected into a Quench column (not shown) via a pipe (not shown), preferably heated to ensure the minimum temperature is maintained.

The elevated temperature of the gas from the exhaust outlet is then rapidly quenched with water in the Quench Column. The rapid cooling helps to prevent the formation of unwanted organic by-products.

Preferably a thermal oxidiser in accordance with the present invention is arranged to operate with a constant flow of gas through the combustion gas input lance which is independent of the feed liquid flow rate. The feed gas is made up predominately of oxygen and nitrogen and may be supplemented with a minor proportion of waste gas material, for example collected from the regeneration of molecular sieve traps.

Preferably, the concentration of the oxygen is limited to around 70% (v/v) to help prevent excessive oxidation of the retort 3 at 1150° C. in the Feed Gas Zone. Preferably the flow of oxygen is fixed and is always over 100% (v/v) excess based upon the max flow and calorific conditions of the liquid being oxidised. This will ensure that there is always excess oxygen for all liquid flow conditions to achieve very high destruction efficiencies (and always remain above the Environmental Agency guidance of 6% v/v). The reasons for gas flow being preferably independent of the liquid flow are to simplify the control basis and to ensure there is always turbulent mixing over the salt tray, as turbulent mixing over the salt tray contributes to effective oxidation.

Preferably the feed liquid flow rate to the retort can vary from 0 to 1.5 l/hr. A small constant flow of, for example, 10 l/hr of nitrogen is maintained to the feed liquid input lance 7 by a mass flow controller to help purge the feed liquid input lance of combustible vapours and prevent oxidation in the lance 7.

During use feed liquid waste stream is fed down the feed liquid input lance 7 that is mounted on the top of the Oxidation Zone. The liquid falls under the influence of gravity onto the hot surface of the salt tray 9 where it is vaporised. The benefits of this design are:

1. Any deposits that would remain as solids, or form molten liquids at this temperature, would be retained in the salt tray 9, reducing corrosion of the retort. 2. Vaporised organics would rapidly oxidise at the elevated temperatures in the Oxidation Zone. 3. In the present invention, the inside diameter of feed liquid input lance 7 is preferably greater than 3 mm and most preferably greater than 6 mm. This is larger than the nozzles on an atomiser; therefore it is unlikely to block—which would result in a rapid decrease in performance. 4. Changes in viscosity or density of the fluids will not have a major effect on the performance of this liquid feed system. If, however, as is common in the prior art an atomiser would have been used, an atomiser would have been sensitive to this. 5. Deposits that are likely to form in the Oxidation Zone are trapped on a sacrificial surface, i.e. the salt tray 9, which can be removed relatively easily. 6. The system has simplicity as an advantage. 7. This system easily handles a wide range of materials that could form deposits.

Preferably the salt tray 9 is formed to maximise the surface area for vaporising the liquid feedstock while capturing the inorganic salts.

The feed liquid input lance is preferably designed to deliver the feed liquid into the salt tray while minimising the vaporisation of volatile compounds in the feed liquid while in the lance. The lance is preferably liquid cooled.

The feed liquid input lance is mounted in the vertical orientation to help prevent sagging over time in the high temperature environment. An insulated cover on the feed liquid input lance is preferably provided. The insulation cover has two purposes:

1. To increase the outer surface temperature of the feed liquid input lance assembly so that condensation of inorganic compounds on the lance is reduced. 2. To provide added protection to the welds on the cooling water section of the lance from attack by corrosive components. The design of the insulation cover preferably allows it to form a protective shield over the feed liquid input lance welds.

Preferably the tip of the feed liquid input lance protrudes into the retort 3 a distance of from 5 mm to 25 mm, more preferably between 10 and 20 mm.

Preferably the combustion gas input lance 5 is arranged to provide a high velocity (and hence turbulence) over the salt tray 9. The turbulence generated in the Oxidation Zone ensures good mixing (as required by EA guidance) and subsequent high conversion of the feed stock. Preferably the combustion gas input lance has an opening diameter of between 2 and 10 mm, more preferably between 5 and 8 mm and preferably 7 mm.

Preferably heat shields are provided on the combustion gas input lance 5 to help ensure excessive radiation to its entry flange on the retort end plate 17 can be avoided.

Decomposition of organic compounds involves complex chain reactions. Established theory has shown that the extent of the destruction of organic compounds is dependant upon holding the components for a residence time at high temperatures in the presence of oxygen.

The residence zone is preferably designed to achieve a residence time of over 4 seconds based upon a laminar flow profile under maximum flow conditions. However, with the use of the flow straighteners (which disrupt the laminar flow), the actual residence time will be significantly greater than this value and will significantly exceed the minimum requirement of 2 seconds which is an Environmental Agency guideline.

While the present invention has been illustrated by an example of an embodiment with one feed liquid input lance it is conceivable to provide a plurality of feed liquid input lances. A plurality of feed liquid input lances could be useful to ensure complete vaporisation of the feed liquid at high feed liquid flow rates.

The above examples illustrate specific aspects of the present invention and are not intended to limit the scope thereof in any respect and should not be so construed. Those skilled in the art having the benefit of the teachings of the present invention as set forth above, can effect numerous modifications thereto. These modifications are to be construed as being encompassed within the scope of the present invention as set forth in the appended claims. 

1. A thermal oxidiser (1) for the oxidation of a feed liquid (W) comprising: an elongated retort (3) provided with a combustion gas input lance (5), a feed liquid input lance (7), one or more flow straighteners (11) and an exhaust gas outlet (13); wherein said retort (3) includes an elongated wall (15) with a first end plate (17) fastened to a first end (19) thereof and a second end plate (21) fastened to the second, opposite end (23) thereof, the combustion gas input lance (5) projects substantially horizontally through first end plate (19) into said retort (3); and the feed liquid input lance (7) projects downwards through the retort wall (15) into said retort (3).
 2. The thermal oxidiser of claim 1, further comprising a salt tray (9) positioned below feed liquid input lance (7).
 3. The thermal oxidiser of claim 1, wherein said combustion gas input lance (5) is adapted to mix waste gas material with oxygen and nitrogen.
 4. A method for the oxidation of a feed liquid (W) comprising: feeding said feed liquid (W) into a thermal oxidiser (1) through a feed liquid input lance (7); providing through a combustion gas input lance (5) more than sufficient oxygen to oxidise said feed liquid (W); combusting said feed liquid (W) at a destruction temperature sufficient to ensure oxidation of the feed liquid constituents to their most oxidised state; maintaining the combustion products at said destruction temperature for at least 2 seconds; and exhausting said combustion products from said thermal oxidiser (1). 